Solar ultraviolet (UV) radiation is a proven mutagen, toxicant, and carcinogen; little is known, however, about the underlying chemical processes causing these effects. We have recently established that reactive species of oxygen are generated by solar UV from endogenously occurring cellular photosensitizers in photodynamic-type reactions and are important intermediates in the biological effects of these radiations, especially at wavelengths longer than about 320 nm. The identities of these species and the mechanisms through which they exert biological effects are not known. We propose to combine the chemical expertise in reactive oxygen species chemistry, biochemistry, and methodology of C.S. Foote and N. I. Krinsky with the photobiological expertise of M. J. and J. G. Peak to elucidate the nature and mechanisms of action of the specific reactive oxygen species generated. These experiments, using mutagenesis in E. coli and damage in isolated bacterial DNA as end points, will provide insights into the basic molecular mechanisms whereby these reactive intermediates exert their biological effects. These insights may provide information relevant to our understanding of a possible role of the reactive oxygen species in carcinogenesis, since solar UV radiations are the main etiological factor in the induction of more than 70% of all human cancers. This information will also improve our understanding of hazards caused by the possible enhanced fluxes of solar UV radiation reaching the earch as a consequence of the attenuation of our stratospheric ozone shield.